Add backlight control for Litra Beam LX

[Copilot]

Litra Beam LX has a separate RGB backlight that was not controllable through this driver. This adds full support for the backlight independent of the main light.

Driver API

Added six functions to driver.ts:

setBacklightOn(device)
setBacklightOff(device)
toggleBacklight(device)
isBacklightOn(device)
setBacklightBrightnessPercentage(device, percentage)
getBacklightBrightnessPercentage(device)

All functions validate device type and throw if called on non-LX devices.

CLI Commands

• litra-back-on / litra-back-off / litra-back-toggle
• litra-back-brightness [percentage] - get or set brightness (0-100)

All support --serial-number for multi-device setups.

HID Protocol

Uses LX-specific command bytes:

• On/Off: 0x11, 0xff, 0x06, 0x1d, 0x00|0x01
• State query: 0x11, 0xff, 0x06, 0x11
• Brightness set: 0x11, 0xff, 0x06, 0x21, percentage
• Brightness get: 0x11, 0xff, 0x06, 0x12

Usage

import { findDevice, setBacklightOn, setBacklightBrightnessPercentage } from 'litra';

const device = findDevice();
setBacklightOn(device);
setBacklightBrightnessPercentage(device, 75);

Original prompt

The Litra Beam LX device supports backlight control functionality that is not currently implemented in this driver. While the device type (DeviceType.LitraBeamLX) is already supported with power, brightness, and temperature controls, the backlight feature needs to be added.

Issue created by Visual Studio Copilot

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